This invention relates to apparatus for detecting charged particles and in particular to apparatus for detecting charged particles in a gaseous environment. The invention also relates to a scanning electron microscope incorporating such apparatus.
There are occasions on which it is necessary to hold specimens in a gaseous environment during imaging in a scanning electron microscope. Examples include imaging of biological or non-conductive specimens. The gaseous environment inhibits the evaporation of moisture from the biological specimens and dissipates surface charges from the nonconductive specimens, which charges would otherwise accumulate to the detriment of image resolution.
Use of a gaseous environment to amplify a secondary electron signal obtained during imaging of a specimen is described in U.S. Pat. No. 4,785,182 (Mancuso et al.), European Patent EP 330 310 (Electroscan Corp.) and PCT Application No. PCT/GB97/03136, in which secondary electrons released by the specimen as a result of interaction with a scanning electron beam are accelerated through the gaseous environment, some of the secondary electrons colliding with the gas molecules making up the gaseous environment. The collisions ionise the gas molecules to release further electrons, some of which collide with other gas molecules to release yet further electrons. The avalanche of electrons released by the ionisation of the gas molecules as a result of such collisions in effect provides the amplification of the secondary electron signal. PCT/GB97/03136 describes apparatus which detects the amplified secondary electron signal by using a photodetector and photomultiplier to detect photons emitted as a result of the collisions of the electrons with gas molecules.
However, as the gas pressure is increased, the strength of the secondary electron signal is reduced since the gas reduces the proportion of the primary beam which reaches the specimen. This correspondingly limits the maximum gas pressure at which the microscope can create an image.
In scanning electron microscopes in which the specimens are held in a substantially evacuated environment during imaging, it is well known to detect the secondary electrons using an Everhard-Thornley detector. In such a detector the secondary electrons are accelerated towards a scintillator to convert them into photons which are transmitted to a photomultiplier.
The Everhard-Thornley detector has been found to be relatively sensitive, but uses a high voltage to establish an electrostatic field in which the secondary electrons are accelerated towards the scintillator. In a gaseous environment, too high a voltage would be sufficient to cause a discharge through the gas.
According to a first aspect of the invention, there is provided apparatus for detecting charged particles, the apparatus comprising a chamber for receiving said particles and being such that, in use, at least a partial vacuum is maintained in the chamber; an impact responsive sensor for detecting particles incident thereon, at least the part of the sensor on which the particles are incident being situated in the chamber; accelerating means for providing, in the chamber, an electric field for accelerating charged particles therein towards the sensor and electrically conductive barrier means sealing the chamber in such a way as to prevent the passage of gas into the chamber, to allow said partial vacuum to be maintained, the barrier means being sufficiently thin to enable the charged particles to be detected to travel therethrough, and being electrically isolated from the accelerating means so as to be capable of being maintained at a different potential from the latter.
Thus, if the apparatus is used in a gaseous environment, the accelerating means can generate a large electrostatic accelerating field without causing an electrical discharge in the gas in the environment as the barrier means, when connected to a suitable voltage source, prevents a field of the same magnitude passing through the barrier means. The large accelerating field makes the apparatus very sensitive to said particles.
Furthermore, where a secondary electron signal from a specimen in the chamber of a scanning electron microscope is amplified by collisions with gaseous molecules in a gas amplification collision zone, the number of the electrons produced as a result of this process is far greater than the number of photons emitted by the collisions within the gas. Since the apparatus is sensitive to charged particles, such as electrons, it is especially suitable for detecting a gas amplified secondary electron signal. In addition, the apparatus can be used to detect a secondary electron signal generated in a vacuum, since secondary electrons generated under those conditions can be accelerated towards the inlet by the application of a suitable voltage.
Preferably, the accelerating means comprises an electrically conductive member situated on or adjacent the sensor, and means for connecting said member to an accelerating voltage.
Preferably, the sensor comprises a scintillator for emitting light in response to the impact of a charged particle therewith.
Preferably, the scintillator incorporates said electrically conductive member.
The sensor may conveniently comprise an Everhard-Thornley detector.
The barrier means conveniently comprises a membrane of metallic foil, preferably aluminum.
An aluminium foil is both cheap and a good conductor of electricity, making it particularly suitable for use as a membrane. Furthermore, due to the low atomic number of the atoms constituting the foil, electrons with a kinetic energy of about 500 eV may penetrate the thin aluminium foil.
Preferably, the aluminium foil is of a thickness of 7.5 nm.
Preferably, the barrier means further comprises support means which extends across said inlet behind the foil to support the latter against pressure exerted on the membrane by gas outside the chamber.
Preferably, the apparatus further includes an electrically conductive cage mounted in front of, but electrically insulated from, the barrier means, the cage being connectable to an accelerating voltage for drawing particles towards the barrier means, the cage being so constructed as to allow the passage of particles therethrough. Where the apparatus is being used as a secondary electron detector, a positive voltage can be applied to the cage to move the gas amplification collision zone in the specimen chamber towards the apparatus, thus further increasing the sensitivity of the sensor.
The apparatus may to advantage include a pump connected to, and operable to evacuate, the chamber, and preferably includes voltage application means for applying a first accelerating voltage to said electrically conductive member and a second accelerating voltage of the same polarity as, but lower than, the first accelerating voltage, to the barrier means.
Preferably, the voltage application means is also operable to apply to the cage a further voltage, of the same polarity as, but lower than, the second voltage.
The cage may to advantage be part-spherical or ellipsoidal.
The invention also lies in a scanning electron microscope having a sample chamber for holding a sample to be imaged in a gaseous environment, generating means for generating a scanning beam of electrons and directing said beam onto a sample in said sample chamber, wherein said chamber also contains detecting means for detecting secondary electrons emitted by the sample, said detecting means comprising apparatus as hereinabove described.
Preferably, the electrically conductive member and barrier means are connected to a voltage application means for applying a voltage of +10 kV to the member and of 0 to +1 kV to the barrier means.
Preferably, the apparatus includes a cage as hereinabove described, the voltage application means being operable to apply a voltage of approximately +300 volts thereto.
The apparatus can be used when there are substantially no gases in the sample chamber since the secondary electrons from the sample will be accelerated towards the barrier means.